1. Field of the Invention
The present invention relates generally to multi-layered golf balls having a core made of a foamed composition. Particularly, the ball includes a dual-layered core having a foam inner core (center) and surrounding outer core layer, preferably made from a thermoset rubber. The ball further includes an inner cover, preferably made from a thermoplastic ionomer composition; and an outer polyurethane cover. The core and inner cover layers have selective weight distribution such that the layers have different densities. The finished ball has good distance and low-spin properties.
2. Brief Review of the Related Art
Both professional and amateur golfer use multi-piece, solid golf balls today. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made of a natural or synthetic rubber such as polybutadiene, styrene butadiene, or polyisoprene. The cover surrounds the inner core and may be made of a variety of materials including ethylene acid copolymer ionomers, polyamides, polyesters, polyurethanes, and polyureas.
In recent years, three-piece, four-piece, and even five-piece balls have become more popular. New manufacturing technologies, lower material costs, and desirable ball playing performance properties have contributed to these multi-piece balls becoming more popular. Many golf balls used today have multi-layered cores comprising an inner core and at least one surrounding outer core layer. For example, the inner core may be made of a relatively soft and resilient material, while the outer core may be made of a harder and more rigid material. The “dual-core” sub-assembly is encapsulated by a cover of at least one layer to provide a final ball assembly. Different materials can be used to manufacture the core and cover and impart desirable properties to the final ball.
The core sub-assembly located inside of the golf ball acts as an engine or spring for the ball. Thus, the composition and construction of the core is a key factor in determining the resiliency and rebounding performance of the ball. In general, the rebounding performance of the ball is determined by calculating its initial velocity after being struck by the face of the golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “Coefficient of Restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under such conditions. Balls with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance as opposed to balls with low COR values. These properties are particularly important for long distance shots. For example, balls having high resiliency and COR values tend to travel a far distance when struck by a driver club from a tee.
The durability, spin rate, and feel of the ball also are important properties. In general, the durability of the ball refers to the impact-resistance of the ball. Balls having low durability appear worn and damaged even when such balls are used only for brief time periods. In some instances, the cover may be cracked or torn. The spin rate refers to the ball's rate of rotation after it is hit by a club. Balls having a relatively high spin rate are advantageous for short distance shots made with irons and wedges. Professional and highly skilled amateur golfers can place a back spin more easily on such balls. This helps a player better control the ball and improves shot accuracy and placement. By placing the right amount of spin on the ball, the player can get the ball to stop precisely on the green or place a fade on the ball during approach shots. On the other hand, recreational players who cannot intentionally control the spin of the ball when hitting it with a club are less likely to use high spin balls. For such players, the ball can spin sideways more easily and drift far-off the course, especially if it is hooked or sliced. Meanwhile, the “feel” of the ball generally refers to the sensation that a player experiences when striking the ball with the club and it is a difficult property to quantify. Most players prefer balls having a soft feel, because the player experience a more natural and comfortable sensation when their club face makes contact with these balls. Balls having a softer feel are particularly desirable when making short shots around the green, because the player senses more and can place a better touch on such balls. The feel of the ball primarily depends upon the hardness and compression of the ball.
Manufacturers of golf balls are constantly looking to different materials and ball constructions for improving the playing performance and other properties of the ball. For example, golf ball manufacturers have looked at ball constructions, wherein the density or specific gravity among the multiple layers of the golf ball is adjusted to control its spin rate. The total weight of a golf ball preferably conforms to weight limits set by the United States Golf Association (“USGA”). Although the total weight of the golf ball is controlled, the distribution of weight within the ball can vary. Redistributing the weight or mass of the golf ball either towards the center of the ball or towards the outer surface of the ball changes its flight and spin properties.
For example, the weight can be shifted towards the center of the ball to increase the spin rate as described in Yamada, U.S. Pat. No. 4,625,964, wherein the golf ball contains a core, an intermediate layer, and a cover, and the core has a specific gravity of at least 1.50 and a diameter of less than 32 mm, the intermediate layer having a lower specific gravity than the core. Chikaraishi et al., U.S. Pat. No. 5,048,838 discloses another three-piece golf ball containing a two-piece solid core and a cover. The dense inner core has a diameter in the range of 15-25 mm with a specific gravity of 1.2 to 4.0 and the outer core layer has a specific gravity of 0.1 to 3.0 less than the specific gravity of the inner core. Gentiluomo, U.S. Pat. No. 5,104,126 discloses a ball with a dense inner core made of steel, lead, brass, zinc, copper, and a filled elastomer, wherein the core has a specific gravity of at least 1.25. The inner core is encapsulated by a lower density syntactic foam composition and this construction is encapsulated by an ionomer cover. Yabuki et al., U.S. Pat. No. 5,482,285 discloses a three-piece golf ball having an inner core and outer core encapsulated by an ionomer cover. The specific gravity of the outer core is reduced so that it falls within the range of 0.2 to 1.0. The specific gravity of the inner core is adjusted so that the total weight of the inner/outer core falls within a range of 32.0 to 39.0 g. In other instances, the weight can be shifted to the outside portion of the ball and away from the center. For example, Sullivan and Nesbitt, U.S. Pat. No. 6,120,393 discloses golf balls having a low spin rate. The balls comprise a relatively soft, multi-piece core and a hard cover. The inner core is hollow and may be filled with gases, while the outer core layer is made of a soft, resilient material. Sullivan and Nesbitt, U.S. Pat. No. 6,142,887 disclose a golf ball containing a core, a thin spherical layer, and a polymeric outer cover. The thin spherical layer comprises a metal, ceramic, or composite material such a silicon carbide, glass, carbon, boron carbide, and aramid materials.
Golf ball manufacturers also have looked at lighter-weight materials, such as foams, for making the inner core. For example, Puckett and Cadorniga, U.S. Pat. Nos. 4,836,552 and 4,839,116 disclose one-piece, short distance golf balls made of a foam composition comprising a thermoplastic polymer (ethylene acid copolymer ionomer such as Surlyn®) and filler material (microscopic glass bubbles). The density of the composition increases from the center to the surface of the ball. Thus, the ball has relatively dense outer skin and a cellular inner core. According to the '552 and '116 patents, by providing a short distance golf ball, which will play approximately 50% of the distance of a conventional golf ball, the land requirements for a golf course can be reduced 67% to 50%.
Gentiluomo, U.S. Pat. No. 5,104,126 discloses a three-piece golf ball (FIG. 2) containing a high density center (3) made of steel, surrounded by an outer core (4) of low density resilient syntactic foam composition, and encapsulated by an ethylene acid copolymer ionomer (Surlyn®) cover (5). The '126 patent defines the syntactic foam as being a low density composition consisting of granulated cork or hollow spheres of either phenolic, epoxy, ceramic or glass, dispersed within a resilient elastomer.
Aoyama, U.S. Pat. Nos. 5,688,192 and 5,823,889 disclose a golf ball containing a core, wherein the core comprising an inner and outer portion, and a cover made of a material such as balata rubber or ethylene acid copolymer ionomer. The core is made by foaming, injecting a compressible material, gasses, blowing agents, or gas-containing microspheres into polybutadiene or other core material. According to the '889 patent, polyurethane compositions may be used. The compressible material, for example, gas-containing compressible cells may be dispersed in a limited part of the core so that the portion containing the compressible material has a specific gravity of greater than 1.00.
Golf ball manufacturers have considered foam inner cores over the years, but they have experienced drawbacks with using such foam materials. For example, one disadvantage with golf balls having a foam core is the ball tends to have low resiliency. That is, the velocity of the ball tends to be low after being hit by a club and the ball generally travels short distances. It would be desirable to have a foam core with improved resiliency that would allow players to generate higher initial ball speed. With higher ball speeds, players can make longer distance shots. Particularly, it would be desirable to develop multi-layered foam core constructions having the proper weight distribution to provide the ball with good distance properties. The present invention provides new foam cores and balls having improved weigh distribution, resiliency and other advantageous properties, features, and benefits.